Different Nbody codes:
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nbody0.f90
           This is the shortest code for leapfrog integration.
           It has less than 40 lines. And it works.
           Note use of f90 constructs: vectors and intrinsic function SUM
           This code is simple, but it does not have all options of other codes.
           

nbody1.f  + initial.f
       Constant-step leapfrog integration and initial conditions.
       This is f77 code.
       Both codes - initial and nbody1 - should be linked with nbodyaux.f.  
       For example, in order to compile nbody1, give the following command
            ifort -O3 nbody1.f nbodyaux.f -o nbody1.exe
       Configuration file is nbody1.h
       You need to have a subdirectory DATA. All the data files 
       should be placed there. 

nbody2.f90 + initial2.f90 + nbody2aux.f90
       Variable-step leapfrog f90 code. It is parallelized with OpenMP
       Parameters are set in module nbody2aux.f90
       Four parameters define accuracy of integration:
            dtstep    -- large integration time-step. Every large step can be split
                         into  smaller number of steps up to 2**(iMultiple).
            iMultiple -- if zero, then only a constant step dt is used for all
                                  particles
                      -- if positive number, it defines the maximum number of particle
                         species. Each specie has the same time-step. Time-steps can
                         change only by factor two from on specie to another.
                         At the beginning of a large step the code decides to which
                         specie each particle belongs to. This decision is made based
                         on (a) particle acceleration and (b) particle speed. Each 
                         condition brings in a free parameter 
            aStep     -- particle displacement dx should be less than aStep
            Alph      -- the rate of change of velocities dV per time-step should be
                         less than dV < Alph*V
        Other parameters:
            eps       -- force softening
            dPrintTime-- frequency of writing snapshots, estimates of energies, and
                         other analysis of the data
            RunTime   -- Duration of simulation
        makefile      -- rules of compilation:
                            make initial2   <= complile and link initial conditions
                            make nbody2     <= compile and link the main code
                            make clean      <= remove executables and object files
        all data files are read and written to subdirectory DATA